We found the source of the smell to be coming from her ears. Through a bit of research, we concluded that conditioner had not been thoroughly washed out from behind her ears and had begun to smell.
THIS IS, IN FACT, NOT THE CASE.
On this trip, my sister's smelling continues. It is not conditioner. It is something far, far more disgusting.
She swims. Some of the water got stuck inside her ear and HAS BEGUN TO ROT.
THE INSIDE OF MY SISTER'S EARS ARE ROTTING.
AND CAUSING A HORRIBLE, HORRIBLE SMELL.
She's got the ear rot.
We had to pour hydrogen peroxide in her ears two days in a row before the smell started to go away.
I did a google image search for "ear rot" and got this: